1. Field of the Invention
The present invention relates to a base station selection system apparatus and method for a wireless LAN which enables a mobile terminal moving on a wireless LAN to quickly switch connection with one base station to another.
2. Prior Art
Recently, as works in office become automated, LANs are increasingly introduced in companies. With this tendency, the number of connection lines used for a premises LAN of a company is increasing. Conventionally, a LAN constructed in a facility is based on a network composed of cables such as coaxial cables or twisted-pair wires. However, people come to regard this as rather irksome, because introduction of a cabled LAN into an office will cause a so-called xe2x80x9cflood of wires,xe2x80x9d and thus if an office automation machine of that office is moved, or the arrangement of furniture of that office is changed, reconnection of the wires will take a undue labor and time. As a solution to such a problem, the wireless LAN attracts the attention of people, because the wireless LAN, a system based on a wireless network, will relieve the user of troubles involved in the reconnection of wires inherent to a conventional, cabled .LAN.
Introduction of a wireless LAN will bring a number of merits: an office or factory is relieved of the restrictions imposed by wiring; terminals may be chosen more flexibly than is the case with a cabled LAN; and use of portable personal computers or use in a vehicle will become possible.
With such people""s expectation towards the wireless LAN as a background, IEEE 802.11 working group started to standardize the operating conditions of the wireless LAN particularly with a view to meet the problems involved in physical layers and MAC or media access control for heavy traffic, and set forth an access mode such as CSMA/CA (carrier sense multiple access with collision avoidance).
However, in contrast with communication between mobile terminals on a public network, communication between mobile terminals on a private wireless network such as one implemented in a company has following problems.
With a public communication network for mobile terminals constructed based on a definite design by communication professionals over a wide area working, for example, on a cellular communication mode, communication zones called cells each with a certain area are arranged around base stations in an urban area in a regular manner and with no interstices between adjacent cells.
The data regarding the location of a mobile terminal is registered, together with the subscriber number of the terminal, through a base station to a location database which serves as a center for management of the location data of individual terminals. If a mobile terminal moves from one communication zone to another, a specific locating means tracks the course of the terminal, and updates the location data of that terminal. The detail of the tracking method will not be touched here. If another terminal sends a call for the moving terminal, connection of the two terminals are achieved based on the subscriber number and current location data of the moving terminal registered in this location database.
In contrast, with a private wireless LAN implemented in a company, it is often difficult to arrange base stations in a regular manner on account of the restrictions imposed by the area and shape of the premise of the company. Moreover, it is often required especially in an office or factory to flexibly modify the arrangement of the network according to current given conditions. Thus, demand for the system to enable easy and quick switching of connections based on the location data of mobile terminals dispersed in a certain area is strong.
In other words, demand for the system enabling the following is strong: even in a wireless LAN where the user often exchanges apparatuses serving as base stations for others, or where the apparatuses are often moved from one place to another, the system has a means to enable a terminal to connect instantly with a base station after removal, or to minimize the procedures necessary for connecting the terminal with a base station apparatus even after that base station has been just removed.
For example, the invention disclosed in Japanese Patent Laid-Open No. 7-307972 titled xe2x80x9cMethod for allocating radio channelsxe2x80x9d introduces a method for dynamically allocating voice channels in a digital cordless telephone system based on the voice channel allocation statistics data of communications through the voice channels allocated to mobile stations, and on the data listed in the voice channel allocation table prepared from the measurements of electric field intensity of all the voice channels obtained at regular intervals.
The invention disclosed in Japanese Patent Laid-Open No. 9-135473 introduces a technique for a wireless packet communication system where communication is achieved in a point-to-multipoint manner, whereby it is possible to select an appropriate packet channel from among candidate channels, without resorting to a dedicated receiver. However, this prior art concerns with a technique how to allocate channels when a particular base station communicates with mobile terminals using TDMA mode.
However, the invention disclosed in Japanese Patent Laid-Open No. 7-307972 is provided to allocate radio channels in accordance with current use conditions, and does not give any description as to the method how a new base station should be selected when a mobile terminal enters the communication zone of another base station.
The technique disclosed in Japanese Patent Laid-Open No. 9-135473 does not concern with the method how a new base station should be selected when a mobile terminal enters the communication zone of another base station. Moreover, the means disclosed in those inventions are problematic in that they do not enable a terminal to automatically and efficiently select a frequency appropriate for the base station to be connected next even when base stations have been just removed.
Selection of a base station according to this invention is achieved with a means which enables a terminal to be instantly connected with a new base station even when the terminal moves about in a premise.
Generally, if a terminal moves from the communication zone of one base station into that of another, the mobile station identifies this newly entered base station and uses a frequency band assigned thereto, thereby establishing communication through the newly entered base station. The principal object of this invention is to provide a method for automatically and efficiently switching from a foregoing base station apparatus to a current base station apparatus when the terminal machine moves in a network, based on the data provided by terminal machines, and a means for managing the data useful for the above purpose.
The object of this invention is achieved by providing a communication system based on a wireless LAN comprising a plurality of wireless base stations, one or plural wireless mobile terminals to communicate with each other using specific frequency bands, and a cabled LAN connecting the plural wireless base stations, wherein:
The wireless base station selecting device of this invention contains a means to summate the statistics data regarding the connection status of individual mobile terminals which move from the communication zones of some base stations to those of other base stations, and a means to transmit base station switching data regarding base stations to be possibly chosen at the next moment arranged in the order of probability of their choice to individual terminal machines, while the wireless mobile terminal contains a frequency selecting means which, the wireless mobile terminal being informed of the base station switching data before it leaves the communication zone of a base station, and arranging the base stations in the order of choice probability based on those previously given data, chooses appropriate frequencies for voice communication in accordance with the base stations arranged as above.